1. Field of the Invention
The invention relates to color electrophotography and, more particularly, to an electrophotographic process, element and apparatus for producing on a single support a self-registered, multicolored image from a single exposure of a color original.
2. Description of the Prior Art
The well known and basic xerographic process can be adapted to produce multicolor reproductions by first color separating the original document to be reproduced into its primary color components red, green and blue. Each color component is then used to record a separate latent electrostatic image on the surface of a photoconductive element. The recorded red, green and blue images are developed using toners containing subtractive colorants that are complements of the primary colors recorded, i.e., cyan, magenta and yellow, respectively. Finally, each developed image is individually transferred from the photoconductive element to a final support material to produce a multicolor reproduction of the original. One disadvantage of this process is that the formation of color separations is an expensive and tedious operation. Another disadvantage is that multiple exposure and transfer operations are required, which operations must be in precise registration in order to produce high quality prints.
A subtractive color electrophotographic process which eliminates the need to form color separations is disclosed in U.S. Pat. No. 2,962,375 to Schaffert. In accordance with the teachings of this reference, a plurality of photoconductive layers, each conductively sensitive to light of a different primary color are superimposed one on top of the other. A light image of a color original is projected onto the array of photoconductors from one side thereof so that the wavelengths of the exposing light selectively pass through the first of the photoconductors in reaching the last of the photoconductors. An electric field is applied through the photoconductive layers while they are exposed in this manner so as to produce a plurality of developable electrostatic latent images which are developed with a toner of the color complementary to the primary light of the exposure. While the Schaffert process requires only a single exposure of the original, registration is still a problem since each of the developed images must still be individually transferred to a final receiving support.
U.S. Pat. No. 3,702,483 to Fantuzzo discloses an electrophotographic method for producing a color reproduction of an original employing a single exposure and a single transfer operation. In this method, a color original is exposed to a uniformly charged photoconductive layer. The original contains colors having reflective characteristics that are capable of selectively discharging the photoconductive surface to discrete charge density levels so that each color is recorded thereon at a discrete charge potential. The photoconductive layer is then transported through a developing station containing a plurality of developing units equal in number to the number of discernible colors in the original. Each developing unit has an electrical control means associated therewith so that only color components recorded at or above a predetermined potential are developed within the unit. The recorded images are developed in a descending order of magnitude and the final color rendition is then transferred in a single operation to a sheet support material. One disadvantage of the Fantuzzo method is that it cannot faithfully reproduce originals comprised of colors which reflect or transmit similar quantities of light onto the photoconductor. Another disadvantage is that some residual voltage remains in the developed regions which, in some cases, may be sufficient to attract additional toner in subsequent developing units, thereby altering the color balance of the reproduction.
Another electrophotographic method for producing a color reproduction of an original which requires only a single exposure and a single transfer operation is discloses in U.S. Pat. No. 3,836,363 to Plutchak. The Plutchak reference utilizes a recording element comprising a multicolor additive filter mosaic sandwiched between two pan-sensitive photoconductive layers. To carry out the process, one photoconductive layer of the recording element is uniformly charged and imagewise exposed to a color original through the mosaic. The resulting electrostatic latent image is developed with an opaque toner. The other photoconductive layer is then uniformly charged, exposed through the opaque toner bearing surface of the first photoconductive layer and the mosaic, and developed three successive times using red, green and blue exposing lights and cyan, magenta and yellow toners, respectively. The resulting color image is then transferred to a final receiver sheet. One disadvantage of the Plutchak reference is that a high quality filter mosaic is required which increases the manufacturing costs of the recording element. Furthermore, use of a mosaic requires controlled smearing of either the exposure or transfer operation of the process in order to produce a faithful color reproduction.